1. Field of the Invention:
The present invention relates to silicon gyros of the type in which rotation rate is measured by the Coriolis effect-induced deflection of a sensor element that includes a paddle that is supported by aligned flexures defining an axis of rotation. In particular, the present invention pertains to such a device that includes wafer elements which integrate driving and sensing functions.
2. Description of the Prior Art
Precision micro-mechanical devices have wide application in the fields of inertial navigation and guidance with respect to both long-range, re-usable vehicles, such as aircraft, and relatively short-range, one-use vehicles, such as munitions. Such devices may be employed to measure acceleration directly and rotation rate indirectly through the Coriolis principle. According to that principle, a body traveling at a velocity {overscore (V)}in a coordinate frame which is subject to rotation {overscore (xcexa9)}experiences an acceleration {overscore (A)}c defined as the cross product {overscore (A)}c =2{overscore (xcexa9)}xc3x97{overscore (V)}. By imposing a sinusoidal relative velocity of the form:
{overscore (V)}={overscore (V)}osin xcfx89t
The corresponding Coriolis acceleration then becomes:
{overscore (A)}c=2{overscore (xcexa9)}xc3x97{overscore (V)}o sin xcfx89t
The measurement of rotation rate is obtained by determining the resultant sinusoidal Coriolis force exerted upon a deflectable force sensitive member.
Micromechanical devices are well suited for operation in low cost systems due to the compactness, simplicity and batch processing capabilities that they offer. One type of micromechanical inertial sensor employs a paddle that is rotatable about an axis defined by aligned flexure beams that support it with respect to a counter-oscillating hub (rotation rate sensor element). Pending U.S. patent application Ser. No. 09/127,375 of inventor Stanley F. Wyse entitled xe2x80x9cMicromachined Rotation Sensor with Modular Sensor Elementsxe2x80x9d and 08/903,499 of Robert E. Stewart and Stanlet F. Wyse entitled xe2x80x9cNavigation Grade Micromachined Rotation Sensor Systemxe2x80x9d disclose inertial sensors of the foregoing type.
Devices of the above-identified type typically comprise stacks of silicon wafers. One of such wafers, known as a driver wafer, includes a plurality of radially-directed electrodes formed upon one or both of its opposed surfaces. A second wafer includes a plurality of radially-directed electrodes formed upon a facing surface in an assembled device. In some embodiments, this wafer is known as a driven wafer. The electrodes of the driven wafer are offset from those of the driver wafer (which is mounted to the case that surrounds the sensor) so that, upon energization, the driven element will be caused to oscillate in response to an a.c. voltage signal applied to the driver electrodes. A sensor wafer that includes the paddle is fixed to the driven wafer in such a way that the paddle is caused to oscillate at the chosen dither frequency causing out-of-plane oscillations of the paddle with respect to the sensor wafer. Such out-of-plane oscillations of the paddle are detected to provide the Coriolis acceleration that is readily converted to rotation rate.
As an alternative, the above patent applications also teach arrangements employing a pair of driver wafers, each having a set of radially-directed electrodes on facing surfaces offset from one another.
Additional electrodes are provided for torquing and picking-off the rotation of the paddle about the axis formed by aligned central flexure beams. In pending patent application 09/127,375, such electrodes are fixed to cover wafers adjacent opposed surfaces of the sensor wafer.
FIG. 1 is a side elevation view in cross-section of a device in accordance with the above-described prior art. As can be seen, the device comprises a sensor stack 10 comprising a top cover wafer 12 having vias 14 and 16 defined therein for contacting electrodes 18 and 20 respectively of an electrode layer 22 that includes a surrounding guard ring 24. The wafers 12 and 22 are fusion-bonded to one another at an oxide layer 26. A bottom cover wafer 28 is configured similar to the top cover wafer 12 and is indirectly fusion-bonded to a lower electrode wafer 30 at an oxide layer 32 to form a like structure.
A sensing element wafer 34 is etched to define a sensing paddle 36 that is supported by aligned flexure beams (one of which is shown at 38) for joining it to a surrounding frame 40. Overlying and underlying oxide layers 42 and 44 respectively are provided for fusion-bonding of the opposed surfaces of the sensing element wafer 34 to the above-described structures.
The above-described sensor stack 10 is fusion-bonded to a dither drive stack 46 at an oxide layer 48. The dither drive stack 46 consists of a driver wafer 50 which, as described above, includes a plurality of radially-arranged electrodes 51 at its top surface, and a driven element wafer 52 that is indirectly fusion-bonded to (a hub of) the driver wafer 50 at an oxide layer 54. The driven wafer 52 includes a set of radially-arrayed electrodes 55 fixed to its lower surface that faces the set of offset electrodes fixed to the top surface of the driver wafer 50. The wafer 52 includes a central hub 56 that is fusion bonded to the driver wafer 50 and an outer member 58 that is bonded to the bottom cover wafer 28 and joined to the hub 56 by reduced thickness flexure beams 60 and 62.
In operation, the sensor stack 10 is dithered at about 2 kHz about a vertical axis 64. The driver wafer 50 is stationary, as is the hub 56 of the driven wafer 52. The outer portion 58 of the driven wafer 52, supported by the flexure beams 60 and 62, is free to oscillate. Electrostatic torquing is provided by interaction of the sets of offset (by 1/4 cycle) electrodes 51, 55.
The device illustrated in FIG. 1, which requires a high vacuum environment to run at high Q, is assembled by bonding the sensor stack 10 to the dither stack 46. This is done by carefully wicking-in EPOXY or like adhesive. Unfortunately, EPOXY outgassing can degrade the quality of the vacuum and, thus, the Q of the device.
As can be seen in FIG. 1 the gap between the driven wafer 52 and the driver wafer 50 extends to the edges of the chips. Such a structure necessitates the use of special and costly dicing techniques to prevent breakage of the dither beams 60 and 62 during manufacture as well as requiring special techniques to keep particles out of the gap as electrostatic forces make the gap attractive to particles that can interfere with dither motion, generate noise and, in most cases, prevent it altogether.
The prior art device requires five (5) silicon wafers, eighteen (18) different masks and the routing of wires from the bottom of the sensor stack 10 through grooves (not shown) in the driven wafer 52. Accordingly, assembly is very time consuming, requiring a degree of hand skill unsuitable for large scale production.
The preceding and other disadvantages of the prior art are addressed by the present invention that provides a rotation sensor. In a first aspect, such sensor includes a first generally-planar wafer that includes a paddle and a plurality of driven elements defined at its opposed sides. A second generally-planar wafer has a plurality of driver electrodes defined on a first surface and a third generally-planar wafer has a plurality of driver electrodes defined on a first surface. The first wafer is arranged relative to the second and third wafers so that the first surfaces of the second and third wafers face the opposed surfaces of said first wafer. A first pair of electrodes is defined on the first surface of the second wafer and is substantially aligned with the paddle. A second pair of electrodes is defined on the first surface of the third wafer and is substantially aligned with the paddle.
In a second aspect, the invention provides a rotation sensor that includes a first generally-planar silicon wafer that comprises a frame surrounding a substantially-round paddle. The first wafer further includes a plurality of driven elements defined at its opposed sides. A second generally-planar silicon wafer has a plurality of driver electrodes defined on a first surface. A third generally-planar silicon wafer has a plurality of driver electrodes defined on a first surface. The first wafer is arranged with respect to the second and third wafers so that the first surfaces of the second and third wafers face the opposed surfaces of the first wafer. A first pair of electrodes is defined on the first surface of the second wafer substantially aligned with the paddle and a second pair of electrodes is defined on the first surface of the third wafer substantially aligned with the paddle.
The preceding and other features and advantages of this invention shall become apparent from the detailed description that follows. Such written description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention. Like numerals refer to like features of the invention throughout both the drawing figures and the written description.